Long term evolution (LTE) based on 3rd generation partnership project (3GPP) technical specification (TS) release 8 is a promising next-generation mobile communication standard.
As disclosed in 3GPP TS 36.211 V8.7.0 (2009-05) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, a physical channel of the LTE can be classified into a downlink channel, i.e., a physical downlink shared channel (PDSCH) and a physical downlink control channel (PDCCH), and an uplink channel, i.e., a physical uplink shared channel (PUSCH) and a physical uplink control channel (PUCCH).
The PUCCH is an uplink control channel used for transmission of an uplink control signal such as a hybrid automatic repeat request (HARQ) positive-acknowledgement (ACK)/negative-acknowledgement (NACK) signal, a channel quality indicator (CQI), and a scheduling request (SR).
Meanwhile, 3GPP LTE-advanced (A) which is an evolution of 3GPP LTE is under development. Examples of techniques employed in the 3GPP LTE-A include carrier aggregation and multiple input multiple output (MIMO) supporting four or more antenna ports.
The carrier aggregation uses a plurality of component carriers. The component carrier is defined with a center frequency and a bandwidth. One downlink component carrier or a pair of an uplink component carrier and a downlink component carrier is mapped to one cell. When a user equipment receives a service by using a plurality of downlink component carriers, it can be said that the user equipment receives the service from a plurality of serving cells.
In a multiple carrier system, a user equipment (UE) does not always receive a service from a plurality of serving cells. The serving cell may be added or deleted according to a service situation. With a change in the number of serving cells, a configuration of a serving cell may be mismatched between the UE and a base station (BS).
An HARQ error is one of problems which occur when the configuration of the serving cell is mismatched. For example, the UE sends HARQ ACK/NACK for one serving cell, but the BS expects HARQ ACK/NACK for a plurality of serving cell. The HARQ error may cause a data loss or a service delay.
Accordingly, there is a need for a method of reducing an HARQ error in a multiple carrier system using a plurality of serving cells.